The magnetic properties of many ferromagnetic materials undergo changes with stress. For example, the magnetic permeability of nickel-iron alloys and iron-cobalt alloys increases and that of nickel decreases with tensile stress. Applied stress can also change the direction of magnetization in magnetic materials. Conversely, if these metals are subject to magnetic fields, their dimensions can change. These magnetostrictive effects, including the Joule effect (change in length when a ferromagnetic rod is placed in a longitudinal field), the Villari effect (change in magnetization when a magnetized ferromagnetic rod is subjected to longitudinal stress), the Matteucci effect (change in magnetization when a ferromagnetic material is subjected to a torque), and the Wiedemann effect (torque on a ferromagnetic cylinder when subjected to a helical magnetic field) can be used for a variety of applications. Examples of the use of ferromagnetic materials include sensors, transducers, and vibrators.
Magnetoelastic stress sensors operate on the principle that magnetic properties of materials are altered by stress via the magnetoelastic coupling. These magnetic property changes can be detected remotely, for example, by measuring magnetic field near the sensor surface using a Hall effect device. Magnetoelastic materials therefore offer realistic prospects for development of contactless sensors for use in stress and torque applications. Magnetostrictive cobalt ferrite composites hold promise for use in advanced magnetomechanical stress and torque sensors because of their high level of magnetostriction, high rate of change of magnetostriction with applied magnetic field (i.e. dλ/dH) and high rate of change of magnetization with applied stress (i.e. dM/dσ).
Most stress sensor applications ideally require materials which exhibit large reversible changes in magnetization with applied stress together with minimal magnetomechanical hysteresis. As described in U.S. Pat. Nos. 6,093,337 and 6,352,649, hereby incorporated by reference, metal bonded cobalt ferrite composites have been shown to be excellent candidates for stress sensors due to a large magnetomechanical effect and high sensitivity to stress. The composites show linear magnetostrictive strains of magnitude up to 225×10−6 with a rate of change of magnetostriction with applied field (dλ/dH)max of 1.3×10−9 A−1m under no external load. They also show good mechanical properties, excellent corrosion resistance, and can be made at low cost.
A drawback to metal-bonded cobalt ferrite composite materials is that the materials exhibit some magnetomechanical hysteresis at room temperature. For these materials to be suitable for sensor applications, the hysteresis must be reduced.
The invention provides such a reduction in hysteresis. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.